1. Field of the Invention
This invention relates to multiple concurrent writeable file systems.
2. Description of the Related Art
A file system provides a structure for storing information, for example application programs, file system information, other data, etc. (hereinafter collectively referred to as simply data) on storage devices such as disk drives, CD-ROM drives, etc. One problem with many file systems is that if the file system is damaged somehow, a large quantity of data can be lost.
In order to prevent such loss of data, backups are often created of a file system. One very efficient method for creating a backup of a file system is to create a snapshot of the file system. A snapshot is an image of the file system at a consistency point, a point at which the file system is self-consistent. A file system is self-consistent if the data stored therein constitutes a valid file system image.
In some file systems, for example Write Anywhere File System Layout (WAFL) file systems, a snapshot of a file system can be created by copying information regarding the organization of data in the file system. Then, as long as the data itself is preserved on the storage device, the data can be accessed through the snapshot. A mechanism is provided in these file systems for preserving this data, for example through a block map.
Conventionally, snapshots are read-only. A read-only snapshot can be used to recall previous versions of data and to repair damage to a file system. These capabilities can be extremely useful. However, these types of snapshots do not provide certain other capabilities that might be useful.